A number of metals and alloys are produced by carbothermic reduction in electric smelting furnaces. Examples of such metals and alloys are silicon and ferrosilicon, ferromanganese, ferronickel, ferrochromium, ferrophosphorus, ferrovanadium and pig iron. These processes require high amounts of electric energy in order to reduce the ore to metals and alloys. In most countries the electricity consumed in the smelting furnaces is mainly produced by combustion of fossil carbon materials resulting in high CO2 emissions.
The reduction materials used in the above processes are coal, coke, charcoal and wood chips. In the reduction process the main part of the carbon in these reduction materials will react with the metal oxides in the ore, but a smaller fraction reacts directly with ambient air. Both reactions emit CO2. The use of fossil carbon materials like coke and coal which are the current predominant sources of reduction material results in a net increase in atmospheric CO2 concentration. However, CO2 released from bio-materials sources like charcoal and wood chips, may be considered carbon neutral if the emission is balanced by growth of new bio-materials that bind an equal share of CO2. Population growth and global warming put pressure on the global society to increase resource efficiency and reduce CO2 emissions.
Currently charcoal production is done by slow pyrolysis or carbonization in typically simplistic kilns or retorts of non-sophistic design. The pyrolysis facility is normally situated far from the plants for carbothermic production of metals and alloys. In addition charcoal production is not very energy efficient and has large emission of harmful particles and PAH (polycyclic aromatic hydrocarbon)-containing gases due to partial or incomplete combustion. Additionally the wide distribution of small charcoal production sites does not make it economically viable to clean off-gases from the charcoal kilns.
It is known to recover energy from off gases from electric smelting furnaces for metals and alloys. The amount of energy that can be recovered by current energy recovery systems is however limited. For instance for a conventional carbothermic silicon furnace the amount of electric energy that can be recovered in an energy recovery system will be in the range of 10-35% of the electric energy supplied to the furnace.
It is therefore a need to improve the energy efficiency of the carbothermic reduction processes for the production of metal and alloys and at the same time reduce CO2 emissions from fossil carbon reduction materials.